<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <script src="https://cdn.bootcdn.net/ajax/libs/vue/2.6.10/vue.js"></script>
</head>
<body>
<div id="app">
    <h1>{{fullName}}</h1>
</div>
<script>
  const vm = new Vue({
    el:"#app",
    data(){
      return{
        firstName:"xiao",
        lastName:"qi",
        fullName:""
      }
    },
    watch:{
      firstName: {
          immediate:true,
          handler(){
              this.fullName = this.firstName.toUpperCase() + " " + this.lastName.toUpperCase()
          },
      lastName: {
          immediate: true,
          handler() {
              this.fullName = this.firstName.toUpperCase() + " " + this.lastName.toUpperCase()
          }
      }
      }
    }
  })
</script>
</body>
</html>